Lossless mp4 is smaller than sum of frames
If what you would like to do is to lower the file size of your images, the best way to do it would be to turn up the PNG compression to 100%. Blender has it at 15% by default. Don't worry, the PNG compression in Blender is lossless, which means that there will be no reduction in image quality whatsoever. The only con is that it may take a few seconds or so longer in order to deal with compression, but I find that the reduced space on the computer is well worth it.
There shouldn't be anything to worry about in terms of compiling images into a video. I would speculate that videos are more optimized for rapid viewing which could be why the files size is smaller than an equal number of videos.